docs: create nph.md (#2536)

docs/contributors/nph.md

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+
+## nph - An opinionated Nim formatter
+This prettifier tool is used to format the nwaku code base.
+
+### VSCode Extension
+https://marketplace.visualstudio.com/items?itemName=arnetheduck.vscode-nph
+
+### GitHub
+https://github.com/arnetheduck/nph
+
+### Installation and configuration
+1. Ask the [nwaku team](https://discord.com/channels/1110799176264056863/1111541184490393691) about the required `nph` version.
+2. Download the desired release from _GitHub_ and place the binary in the PATH env var.
+3. Add the following content into `~/.config/Code/User/settings.json`:
+
+```
+{
+    "[nim]": {
+        "editor.formatOnSave": true,
+        "editor.defaultFormatter": "arnetheduck.vscode-nph"
+    },
+}
+```
+
+With that, every time a Nim file is saved, it will be formatted automatically.
+